Improvement of the Proton Conduction of Copper(II)-Mesoxalate Metal–Organic Frameworks by Strategic Selection of the Counterions

Three copper(II)/mesoxalate-based MOFs with formulas (H3O)[Cu9(Hmesox)6(H2O)6Cl]·8H2O (1), (NH2Me2)0.4(H3O)0.6[Cu9(Hmesox)6(H2O)6Cl]·8H2O (2), and (enH2)0.25(enH)1.5[Cu6(Hmesox)3(mesox)(H2O)6Cl0.5]Cl0.5·5.25H2O (3) were synthesized (H4mesox = mesoxalic acid = 2,2-dihydroxypropanedioic acid, en = ethylenediamine). Essentially, all of the compounds display the same anionic network with a different arrangement of the cations, which have a remarkable effect on the proton conduction of the materials, ranging from 1.16 × 10–4 S cm–1 for 1 to 1.87 × 10–3 S cm–1 for 3 (at 80 °C and 95% RH). These compounds also display antiferromagnetic coupling among the copper(II) ions through both the carboxylate and alkoxido bridges. The values of the principal magnetic coupling constants were calculated by density functional theory (DFT), leading to congruent values that confirm the predominant antiferromagnetic nature of the interactions.

6 Section 2. Single-Crystal structure refinement details Suitable crystals were carefully selected under a polarizing microscope, covered in protective oil and mounted on a 0.05 mm cryo-loop.
Structure data and refinement: Table S1. Crystal data and structure refinement details of compounds 1-3.  7 Section 3. Magnetic Properties Figure S5. Magnetic susceptibility plot as a function of T for 1-3. χ refers to the magnetic susceptibility per mole of Cu(II).

Section 4. DFT calculations Compound 1
We have calculated the magnetic coupling constant between the Cu1 atoms across the alkoxido bridge, J1, and the coupling between Cu1 and Cu2 across the anti-anti carboxylate bridge, J2. For the calculation, we have taken six Cu atoms, six mesoxalate ligands, one chloride ion and six water molecules according to Figure
With those values we calculate the coupling constants J1 and J2 according to the following equations: The coupling through the alkoxido bridge, J1, results in a value of -108.7 cm -1 and the coupling through the anti-anti carboxylate bridge J2 = -76.7 cm -1 .
With the following fraction of the structure of 1, which contains six Cu(II), six mesoxalate ligands and one Cl -, we can calculate the magnetic coupling constants through the alkoxido bridge, J1, and through the chloride bridge, J3, Figure   S7. The values are given below.
We find a slight difference in J1 with respect to the previous calculation, but the difference is small (11cm -1 ), which means that the method is correct and that the set of atoms is not critical for calculating the coupling constants.

Compound 2
We have calculated the J1 and J3 with the same set of atoms shown in Fig   Due to the almost identical structure, we assume that the value of J2 is the same to that found for compound 1.

Compound 3
The structure of 3 contains two different layers that we have labeled as a and b. a layers contain Cu1, Cu2 and Cu3 atoms ( Figure 4)  We have considered much more interesting the study of the b layers that have different connectivity in the carboxylate groups, Figure 7 and Figure S7. J8 and J5 refer to the couplings between Cu5 and Cu6 atoms through alkoxido bridges in the trinuclear entities; J6 to the coupling between Cu6 and Cu4 through the anti-syn carboxylate bridge; and J7 to the coupling between Cu4 and Cu5 through the anti-anti carboxylate bridge. J6 is of special interest since it is the unique anti-syn bridge found in Cu(II) / mesoxalate complexes. Also, J7 is singular since the mesoxalate adopts a μ3-(κO:κO′,κO″:κO″,κO′′:κO″′,κO′′′′:κO″′′) bridging mode with the two alcohol groups deprotonated.  The carboxylate groups adopt an anti-anti connectivity and the magnetic coupling is reinforced by the participation of the two alkoxide groups that reduce the Cu5•••Cu4 distance. 12 E3-E7=2J8 + 4J7 = -581 cm -1 J8= -6.9 cm -1 (Equation S8) The coupling through the alkoxido bridge with a Cu5OCu5 angle of 118.69(2)º, J8, gives a value of -6.9 cm -1 . And the coupling through the anti-anti carboxylate bridge results in a value of J7= -142 cm -1 .
For the calculation of the magnetic coupling between Cu6 atoms through the alkoxido bridge, J5, and the coupling between Cu6 and Cu4 atoms through the anti-syn carboxylate bridge, J6, we have considered the following fragment.
The values are given in Table S6.  E3-E7=2J5 = -12.66 cm -1 J5= -6.3 cm -1 (Equation S10) Finally we have calculated the magnetic coupling through the alkoxido, J1, and alcohol-alkoxide groups, J9, among Cu3 ions in compound 3. The fragment used for this calculation is shown in Figure S10 and the values in the Table S7.   The anionic network has been displayed in grey for clarity. The oxygen atoms of crystallization water molecules and ethylenediammonium cations are located disordered in the pores, and they are represented as "ball and sticks"; e) Same as d, but view along c axis; f) View along c axis for the cavities of 3, where water and ethylenediammonium cations are located. For the picture, only the stacking of layers a, a' and b has been used, for the seek of clarity. Although the water and cations are heavily disordered in the pores and any hydrogen-bonding pathway could be dilucidated, we have drawn the contacts between oxygen atoms of water molecules (depicted in dashed blue lines) with distances O-O between 2.7 and 3.3 Å, typical of donor-aceptor distances in hydrogen bonding, indicating that there is a path for proton conduction.  (14) O2 iv -Cu2-O1W iv 89.91 (15) O3 iii -Cu1-O3 103.6 (2) O2 v -Cu2-O1W iv 89.91 (15) O2 iv -Cu2-O2 v 92.85 (17) O2-Cu2-O1W iv 90.08 (15) O2 iv -Cu2-O2 180.0 O2 vi -Cu2-O1W iv 90.09 (15) O2 v -Cu2-O2 87.15 (17) O3 i -C2-C1 vii 110.5 (3) O2 iv -Cu2-O2 vi 87.15 (17) O1W-Cu2-O1W iv 180.0